Patent Application
20150132590
The present invention is related to a method for coating metal for protecting them against corrosion. The present invention is also related to the coated metal obtained by the method of the invention.
Depending on the application various properties are required from metal surfaces, such as durable corrosion resistance, adherence, stable & aesthetic optical appearance, reflectivity, hydrophobic/hydrophilic, self-healing, anti-fungal, self-cleaning, mechanical surface resistance & flexibility, and possibly others.
The creation of these properties is based on the fundamental understanding of the physical phenomena behind them. For example, durable corrosion protection of a metal can be obtained by shielding it from the environment by a good adhering coating with excellent and durable barrier performance (i.e. no diffusion of water to the metal interface), and ideally—when the coating is damaged somehow—also showing corrosion inhibiting activity through self-healing inhibitor release to re-passivate the underlying metal.
To create or enhance such properties the metal surface undergoes various surface treatments. For most applications, conventional surface processing involves cleaning, etching, metal activation, conversion, primer application before the ultimate finish (mostly painting).
The conventional and still mostly used processing route is long and arduous involving a wide selection of wet chemicals (alkaline and acid), including often still toxic chromium containing species for corrosion inhibition, as well as volatile organic solvents for organic coatings.
In response to this long, sequential processing route involving ecological issues, various alternatives are currently used. Firstly, to avoid the organic solvent issues, waterborne systems are widely accepted. Examples are acrylic and latex based coatings. Secondly, instead of surface conversion followed by a primer, the development of combined systems is quite advanced. An example is silane coating.
Commercial silane solutions are waterborne and do not contain monomer species, therefore, they are considered environmentally sound. Although good alternatives for certain applications and properties, these systems still have their own set of problems. For one, the waterborne systems are much inferior for corrosion protection. The water in the coating is the culprit, as it is one of the essential ingredients for corrosion. Evacuation of the water during polymerisation is not always complete. Aside from poor corrosion properties, this also results in a poorly reticulated coating and can cause problems when the coated metal is exposed to freezing temperatures. Additionally, there are still many separate processing steps involved.
There is a wide interest for developing new multifunctional coatings on metals. The state of the art of research indicates that technologies to deposit ‘mono’functional coatings are known. Some examples: self-cleaning coatings using TiO2, antibacterial using metal nanoparticles (mostly Ag), barrier properties against gas or liquid permeation using SiOx, SiNX or silane coatings, hydrophobic surfaces using fluorinated coatings, non (bio)fouling using plasma-polymerised polyethylene glycol (pp-PEG).
The search for advanced coatings leads to the proposal to synthesize mixed coatings (hybrid coatings) that can combine at least two crucial functional properties in response to the growing demands of the industrial application.
At present, the concept is mostly investigated through wet deposition of waterborne organics mixed with, for example, inorganic nanoparticles, metal ions or other active species.
The addition of corrosion inhibitors to coatings has been a known practice for several decades. However in the prior art, mainly chromium VI (which turned out to be a highly carcinogenic species) was used to obtain self-healing properties.
The working mechanism of inhibitor bearing coatings is based on the corrosion inhibitors being released when the coating is locally damaged, and immediately passivating the metal at the damage site.
Inhibitors are generally classified as anodic-inhibiting the anodic reactions in a corrosion process- or cathodic-inhibiting the cathodic reactions in a corrosion process. So-called multifunctional inhibitors have the ability to inhibit both.
The present invention aims to provide a method for coating a metal substrate for reducing corrosion. More particularly, the method of the invention aims to reduce the number of steps needed to obtain metal item protected against corrosion.
The creation of multifunctional surfaces on metals in an ecologically acceptable way, through an efficient processing is also an aim of this project.
The method of the invention further aims to suppress the need for use of hexavalent chromium in the treatment of metal surfaces.
More generally, the method of the invention aims to replaces the separate conventional wet pre-cleaning, activation, conversion and primer procedures used in prior art for protecting metal item against corrosion.
The present invention is related to a method for protecting a metal substrate against corrosion comprising the step of:
By corrosion inhibitor, it is meant a chemical species which by depositing, absorbing, adsorbing, bonding or reacting with a metal surface, inhibit anodic and/or cathodic electrochemical reactions to occur, which otherwise would result in the unwanted oxidation of the metal.
According to particular preferred embodiment the method of the invention further comprises one or a suitable combination of at least two of the following features:
Another aspect of the invention is related to metal item comprising a corrosion inhibiting coating comprising:
Preferably, said metal item is obtained by the method of the invention.
According to particular preferred embodiment the method of the invention further comprises one or a suitable combination of at least two of the following features:
The present invention is related to a method for protecting metal surfaces against corrosion. In the present invention, an organic coating is performed by a (cold) atmospheric plasma (co-)deposition based on more than one precursor or species: at least one barrier polymer precursor and at least one corrosion inhibitor.
In the field of surface treatment of metallic substrate, a conversion layer is a layer which transforms the metal oxide film into a thin passive film with a thickness of less than 0.1 μm. This passive film consist of oxides and or salts and are formed in a solution by a chemical or electrochemical deposition reaction.
By cold plasma, or non-thermal plasma, it is meant in the present invention a partially ionized gas comprising electrons, ions, atoms, molecules and radicals out of thermodynamic equilibrium characterized by an electron temperature significantly higher than the neutral and ionic species temperature. Preferably, in the present invention, the ionic and neutral temperature (i.e. macroscopic temperature) is lower than 400° C. Advantageously, said neutral and ionic species temperature is lower than 150° C. Ideally, the temperature of neutral and ionic species in the plasma is minimized, lower than 100° C. and/or close to room temperature. The minimization of the temperature of the neutral and ionic species has the technical effect of maintaining large molecular species in the plasma, thereby having a better control of the chemical nature of the deposited layer.
The plasma is also preferably an atmospheric plasma. Advantageously having a pressure comprised between about 1 hPa and about 2000 hPa, preferably between 100 and 1200 hPa, with other ranges obtainable by combining any above specified lower limits with any above specified upper limits being as if explicitly herein written out.
At atmospheric pressure, both the fundamental scientific mechanisms and the required technology are different than for vacuum plasma. Due to the collision impact in the gas phase with electrons or molecules, one deals mostly with radicals and a huge number of fragments. As a consequence, the polymerisation mode is mostly radical-based, with a high degree of cross-linking, leading to potentially very dense polymers, which are of potential interest for good barrier properties, even for very thin coatings.
The final quality of a coating strongly depends on the deposition process itself. Plasma techniques present the advantage that cleaning, activation/functionalisation, deposition and crosslinking can be done “in-situ”, without the requirement of organic solvents or aggressive chemicals.
As it will be shown in the example, the use of low-temperature atmospheric plasma conditions surprisingly maintain the corrosion protective effect of the organometallic inhibitor to be used.
Advantageously, the inhibitor is deliberately located near the metal film interface at a volume concentration much lower than in traditional inhibitor-bearing coatings. This low concentration permits to reduce the consumption of expensive chromium-free ecofriendly inhibitors. As a limited concentration of inhibitor is enough to passivate the metal when a coating is locally damaged, there is no need to distribute the inhibitor throughout the coating thickness. Using atmospheric plasma co-deposition, the inhibitor can be part of a gradient structured coating.
When aluminium is the substrate, the inhibitor is advantageously selected from the group consisting of silicates, phosphates, rare earth metal salts such as CeCl3, Ce(NO3)3, Ce(dbp)3, Ce(dpp)3, La(NO3)3 and Pr(dbp)3, 2,5-dimecapto-1,3,4-thiadiazole, aliphatic mono- and dicarboxylic acids (C6-C10) and a primary aromatic amide, Mercaptobenzothiazole, Mercaptobenzimidazole, Mercaptobenzimidazolesulfate, thiosalicylic acid, quinaldic acid, salicylaldoxime, 8-hydroxyquinoline, Tetrachloro-p-benzoquinone, tannins, and their mixture.
When the substrate is steel, the inhibitor is advantageously selected from the group consisting of silicates, nitrites, phosphates, polyphosphates, phosphonates, rare earth metal salts such as erbium triflates, molybdates, tungstenates, vanadates, zinc cations, borates, tannins, cinnamic acids, alkanoleamines and their mixture.
When the substrate is galvanized steel (and/or zinc), the inhibitor is advantageously selected from the group consisting of silicates, phosphates, molybdates, tungstenates, vanadates, zinc cations, strontium cations, bismuthiol, polycarboxylates, hydroxyl substituted mono- and polyamine, imino derivatives, hydroxylamine derivatives, aliphatic mono- and dicarboxylic acids (C6-C10, such as hexaonic acid) and a primary aromatic amide (such as benzamide), organophosphorous such as 2-phosphonobutane-1,2,4-tricarboxylic acid), polyethylene glycol, tannins and their mixture.
For example, cerium is mined in China and its price has drastically increased once its corrosion-inhibition activity was discovered and published in scientific literature.
Preferably, in order to improve adhesion, And to reduce total consumption of inhibitor, the concentration at the interface between the metal and the coating is preferably reduced. The concentration of the inhibitor is then increased with the distance to the metal interface. The maximum concentration occurs advantageously at a distance to the interface comprised between 1 and 50 nm, preferably between 5 and 25 nm.
As the distance to the metal interface further increases, the concentration of the inhibitor is the advantageously gradually decreased down to zero. This additional barrier layer on top of the active layer reduces the leaching of the inhibitor outside the structure, thereby reducing the total amount of inhibitor used in the structure.
An advantageous feature of plasma co-deposition for producing such structure, is that the gaseous composition feeding the plasma can be dynamically controlled, the time variation of the species concentration determining the spatial gradient in the deposited material.
An advantage of the use of cold plasma, is that it has only limited influence on the intended structure of the inhibitor. This is particularly true when organic or organometallic structures are used.
Preferably, multifunctional corrosion inhibitor are used, with functional groups acting on the cathodic processes and other functional groups acting on the anodic processes.
Known functional groups acting on the cathodic processes are for example rare earth metal salts such as Ce.
Known functional groups acting on the anodic processes are for example phosphate groups.
Advantageously, for the protection of aluminium, Cerium dibutyl phosphate (Ce(dbp)3) having formula:
is used. In this inhibitor, Cerium is active against cathodic corrosion processes and phosphate against the anodic corrosion processes.
The (precursor of the) inhibitor can be introduced in the plasma as a gas, or liquid phase. In case of liquid phase, the introduction can advantageously be performed by spaying an aerosol of liquid droplets directly in the plasma or in a post-plasma area.
The aerosol can preferably comprise an organic solvent and the corrosion inhibitor. This is particularly advantageous when the corrosion inhibitor is solid at ambient temperature, and is soluble in a particular organic solvent. For example, Ce(dbp)3 can be dissolved in dissolved in a methanol:hexane solution.
As a barrier polymer precursor, saturated or unsaturated hydrocarbon (eventually halogenated) can be used. Such precursors give rise to highly hydrophobic coatings preventing water diffusion towards the metal interface.
Preferably, compounds comprising at least one ethylenically unsaturated group can be used. Advantageously, allyl methacrylate is used as the precursor of the organic barrier material. Such precursor produces an efficient organic-based primer type coating exhibiting a good adhesion to the metal substrate and significant barrier properties.
The gradient type coatings according to the invention have shown good adhesion, barrier properties and an autonomous corrosion healing ability by active corrosion inhibitors that passivate the metal in case of coating damage.
The DBD (dielectric barrier discharge) treatments were done in a plasma reactor schematically represented in
The gas injection is done using a toric gas sprinkler placed at the gap between the electrodes, outside the plasma core. The inhibitor solution microdroplets are brought in the same area by means of a glass tube linked to an atomizer as represented in
Allyl methacrylate (98% purity, CAS #96-05-9, Aldrich) has been used as-received, carried by argon (Alphagaz 1, Air Liquide, plasmagen gas) directly into the discharge, during plasma treatment. The coated samples consist of mechanically polished aluminium (AA2024 alloy), 20×30 mm substrates. An AFS-G10S power generator has been used as the plasma source.
In order to control the monomer feed, a part of the plasma gas flow was directed through a bubbler containing allyl methacrylate at room temperature. This flow is referred to as the secondary flow (
The output power was varied from 30 W to 80 W while the operating frequency was fixed to 17.1 kHz. The deposition time was set to 120 s. The total argon flow was always kept constant at 4 L/min. For these experiments, the secondary Ar flow in the bubbler was set to 1 L/min, which corresponds to a monomer feed of approximately 30 mg/min. The surface of the substrate has been pre-treated for 15 s with pure argon plasma (3 L/min) prior to the introduction of the monomer.
This protocol is summarized as follows:
The corrosion inhibitor, namely cerium dibutylphosphate has been dissolved (1% weight) in a methanol:hexane (20:80) solution.
This follows exactly the same protocol as for the deposition of pure AMA which consists of the 15 seconds of cleaning/activation of the substrate and 120 seconds of plasma deposition of AMA. An extra step during the AMA deposition is added to the protocol, involving the inhibitor solution atomizer, and argon, used as carrying gas.
The output power was varied from 30 W to 80 W while the operating frequency was fixed to 17.1 kHz. The TOTAL deposition time was set to 120 s. For these experiments, the secondary Ar flow in the bubbler was set to 1 L/min, which corresponds to a monomer feed of approximately 30 mg/min.
After 15 seconds of the pure AMA deposition, the inhibitor solution microdroplets are carried simultaneously, by argon and through the pyrex tube, directly in the plasma discharge. The extra argon flow was set to 2 L/min, bringing the total argon flow to 6 L/min. The duration of this extra step is 45 seconds. The total amount of inhibitor solution injected in the plasma was 1 mL.
At this point, the injection of the inhibitor was stopped and followed by a 60 seconds deposition of pure AMA in order to form a protective AMA topcoat.
The surface of the substrate has been pre-treated for 15 s with pure argon plasma (3 L/min) prior to the introduction of the monomer.
This protocol is summarized as follows:
The time for the deposition can be varied as desired in order to control the amount of inhibitor injected.
The thickness of the coatings of the two examples have been estimated using Spectroscopic Ellipsometry.
The same protocol as in example 2 was used except that the cerium dibutylphosphate inhibitor was replaced by benzotriazol inhibitor, and the inhibitor was gradually introduced and removed from the plasma, thereby creating a smooth concentration profile.
The same protocol as in example 3 was used, except that the inhibitor was directly injected in step iii, so that no layer without inhibitor was deposited on the substrate.
the Scanning Vibrating Electrode Technique (SVET) was used to demonstrate the corrosion inhibition activity of the coating of the example. This technique is an in-situ local electrochemical technique that allows to measure the corrosion activity above a coated metal while immersed in a corrosive electrolyte. SVET mapping of a plasma coated AA2024 surface, which is an alloy particulary prone to corrosion, shows no corrosion activity during 1 day of immersion in NaCl solution as shown in
Electrochemical Impedance Spectroscopy was also used to verify anti-corrosion activity of the coating. This method is well established to evaluate coatings.
Electrochemical Impedance Spectroscopy is a characterization technique in which a small perturbation voltage over a range of frequencies is applied in an electrochemical system and the response current is measured. The impedance for each measured frequency of the system is calculated. A way to present impedance spectroscopy data is by plotting the real part of the impedance on the X-axis and the imaginary part on the Y-axis of a chart (Nyquist Plot, see
In order to evaluate the efficiency of the inhibitor an artificial scratch has been made in the coatings. This scratch was made in a homemade device in which the same weight was applied in all cases to achieve the same kind of scratch in all cases.
Another two spectra were taken 3 and 23 hours after the scratch.
In order to verify the results the same measurements were performed again and the same influence of the inhibitor in the impedance was observed.
The incorporation of the Ce(dbp)3 as inhibitor inside plasma polymerized coating provides self-healing properties to the coating. Depositing the inhibitor close to the substrate decreases the deposition cost as only a small amount of inhibitor should be used. The efficiency of the inhibitor is still high because it is located close to the substrate.
As can be seen in
| Number | Date | Country | Kind |
|---|---|---|---|
| 12166997.2 | May 2012 | DE | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2013/059496 | 5/7/2013 | WO | 00 |
